Potentiometer means for providing a standardized precision low voltage



L. JULIE 3,454,877 FOTENTIOMETER MEANS FOR PROVIDING A STANDARDIZED July 8, 1969 Sheet of 4 PRECISION LOW VOLTAGE Fi .ed July 2, 1965 INVENTOR Z0555 Jams ATTO R N EYS July 8, 1969 L. JULIE 3,454,377

POTENTI'OMETER MEANS FOR PROVIDING A STANDARDIZED PRECISION LOW VOLTAGE Filed July 2, 1965 Sheet of 4 Q T1 .20 1* q 20 2/ g H F 6% [/62 Von/1 6 F I Sauece 5 7 532 252; 3/ was? [/A/D E? k 23 4 n0 r 7557' 5 40 L /vs aw/v 7' J /9 [MPEDEA/C'E 56 ATTORNEYS y 8 969 L. JULIE 3,454,877

POTENTIOMETER MEANS FOR PROVIDING A STANDARDIZED PRECISION LOW VOLTAGE Filed July '2, 1965 Sheet 5 of 4 a M bl 7112 .7- 6 V62" 53 5 T {a F INVENT R 10:55 414/5 ATTORNEYS 3,454,877 ZED L. JULIE POTENTIOMETER MEANS FOR PROVIDING A STANDARDI July 8, 1969 I PRECISION LOW VOLTAGE Sheet Filed July 2. 1965 INVENTOR Z0595 4/7/4/5 BY az fiwzm w ATTORNEYS United States Patent POTENTIOMETER MEANS FOR PROVIDING A STANDARDIZED PRECISION LOW VOLTAGE Loebe Julie, New York, N.Y., assignor to Julie Research Laboratories, Inc., New York, N.Y., a corporation of New York Filed July 2, 1965, Ser. No. 469,122 Int. Cl. G01r 17/02; H02p 13/00; H02m /06 US. Cl. 324-98 9 Claims ABSTRACT OF THE DISCLOSURE A circuit for producing a high precision low voltage, i.e., microvolt source, is adjustable as to its output. The circuit includes a constant current source and a voltage divider. The voltage divider is characterized by a linear transfer ratio and a constant input impedance.

The instant invention relates to a calibrated potentiometer system for producing a high precision microvolt source and, in particular, the invention herein contemplates the use of a voltage divider characterized by a linear transfer ratio as the variable potentiometer parameter, which divider is combined with preselected calibrating elements in the network to produce a calibrated and low output impedance microvolt source.

Known technology includes various potentiometer circuits designed to provide accurate parameter measurements and standard cell comparison means. In addition, my copending application Ser. No. 308,492, filed Sept. 12, I963, for Precision Potentiometer Circuit and Method for Establishing Same, teaches a precision potentiometer system employing a voltage divider characterized by a linear transfer ratio and a preselected constant input impedance as the variable parameter of the potentiometer system. This system is designed to establish a preselected voltage of one part per million (1 ppm.) accuracy, such as 1.000000 volt across the voltage divider input terminals, whereby calibrated linear values of output voltage from zero to 1.000000 are obtainable at the potentiometer output. The disclosed system employs a certified standard cell, a power source, a galvanometer and standardizing resistors of specified value for calibrating the potentiometer system. The circuit also includes a fixed value resistor having a value one-tenth the input impedance of the divider in series with such divider to provide potentiometer self-calibration to overcome divider scaling errors. Although the system of the copending application provides electrical parameter values of extreme accuracies in the order of one part per million, the design thereof is not entirely satisfactory for achieving comparable accuracies in the very low microvolt ranges. For example, when measuring or calibrating voltages in the range of 0.001 microvolt the known potentiometer systems intrinsically are not suitable because of thermal EMF s originating in the complex circuitry thereof and such phenomena tend to obscure the small values of microvolts being calibrated and measured. The maximum source impedance is at about midpoint on the voltage divider. At this point, the source impedance is about 30,000 ohms. The Johnson noise level resulting from such impedance is of the order of 0.03 microvolt, thereby obscuring the smaller value voltages to be measured.

The instant invention provides a new and improved potentiometer circuit designed for precision microvolt range measurements and calibrations by using a linear transfer ratio voltage divider as the variable parameter of the potentiometer system. In this application, the voltage divider is essentially reversed, wherein a sourceexciting power is fed into the variable arm of the divlder 3,454,877 Patented July 8, 1969 while a load impedance of fixed low impedance value is connected across the input end terminals of the voltage dlvider. The source power is supplied by a constant current generator for driving the potentiometer system.

It is a principal object of this invention to provide method and apparatus for achieving a high accuracy, easily calibrated, low thermal EMF and low output impedance microvolt potentiometer system and measurements.

It is a further object of this invention to provide a method and potentiometer system capable of precision voltage potentials in the microvolt range, wherein the potentiometer output voltage is a calibrated and direct function of a linear transfer ratio, and further where the variable control device of the potentiometer system is a voltage divider characterized by a linear transfer ratio and a fixed value terminal impedance.

It is a further object of the invention to provide means for reducing the effects of thermal EMFs produced by an external temperature difference in the load circuit of a potentiometer system as contemplated herein, and also to provide means for reducing the effects of loading the output of such potentiometer system.

Further objects and advantages will become apparent from the following description of the invention taken in conjunction with the figures, in which:

FIG. 1 is a circuit diagram of a microvolt potentiometer system in accordance with the invention;

FIG. 2 is a circuit diagram of an alternate embodiment of the invention which, in addition, illustrates load compensation means in the output circuit of the system;

FIGS. 2a, 2b and 2c.illustrate schematically the sequence of an alternate embodiment for calibrating and monitoring the standardized element of the potentiometer system;

FIG. 3 is an equivalent T circuit of the voltage divider and load impedance of the potentiometer system;

FIG. 4 is a block diagram illustrating the concept of the instant invention;

FIG. 5 illustrates schematically rnultiwire thermal center of gravity connections between two electrical devices in accordance with the invention;

FIG. 6 is a cross-sectional view of the connection wire array employed in FIG. 5;

FIG. 7 illustrates schematically the use of the thermal center of gravity technique in an output circuit of a potentiometer system in accordance with the invention;

FIG. 8 is a sectional axially elevational view of a thermal gradient reversing polarity switch in accordance with the invention;

FIGS. and 8b are end views of the left and right ends of the switch embodiment of FIG. 8 to permit a comparison of the geometry of one group of switch Wires with respect to the other;

FIG. 9 is a view taken along line 99 of FIG. 8;

FIG. 10 is a view taken along line 1010 of FIG. 8; and

FIG. 11 is a schematic illustration for the purpose of depicting the operation of the switch embodiment of FIG. 8;

FIG. 1 illustrates a potentiometer system 9 in accordance with the invention. The potentiometer system 9 contemplated herein is driven by a feedback type of constant current generator 10. One side of a reference voltage 11 is connected to the input side of a high gain amplifier 12. The output common of amplifier 12 is in series with current regulating resistances 13, 14. The other side of reference voltage 11 connects to resistance 14 at junction 15 to complete the feedback loop in generator 10. Current generator is preset and regulated to provide a fixed value of 10 microamps as the potentiometer source. Adjustable standardizing resistors 16, 17, 18 are Series connected to current generator by the connection at junction The values of the individuals resistors 16, 17, 18 are selected to standardize potentiometer system 9 in accordance with the aforesaid copending patent application.

The potentiometer variable is a voltage divider 19 hav ing at least three terminals defining its input and output and further characterized by a fixed input impedance of known value and a linear transfer ratio K. Terminals 20, 21 are the usual input terminals of such voltage divider. Terminal 22 represents the adjustable arm terminal. Reference is made to said copending application and to United States Patent No. 3,179,880, issued Apr. 20, 1965 for a detailed review of such voltage dividers. The transfer ratio and the output impedance of voltage divider 19 is varied linearly by adjustment of a six digit direct reading dial 23.

A switch 24 is connected in series between standard izing resistor 18 and divider terminal When switch 24 is in its position a, the exciting current of 10 microamps supplied by generator 10 flows through a fixed value 100K resistor 25 to adjustable divider arm terminal 22. Resistor 25 serves to load suitably the output of amplifier 12 when divider slide 22 is at the low end of the divider and at point 21. When switch 24 is in its position b, the generator circuit 10 is connected to divider input terminal 20.

A fixed value resstance 26 is connected as a load across divider terminals 20, 21 by a four terminal yoke connection. The yoke current terminals are 27a and 27b. The yoke voltage terminals are 28a and 28b. A switch 29 is designed to connect yoke terminal 27a to divider terminal 20. Switch 29 is ganged to switch 24. When switch 29 is in its a position, load impedance 26 is connected across the fixed value input impedance of divider 19. When switch 29 is in its b position, load impedance 26 is disconnected from the circuit.

A resistor 30 connects to divider terminals 21. A junction 31 at the other end of resistor 30 connects back to constant current supply 10 at the negative out ut side of amplifier 12. With the divider input impedance (terminals 20 to 21) of a fixed value of 100K ohms, resistor 30 is selected to be 10K ohms and is used in self-calibration of potentiometer circuit 9 for sealing error as described in the aforesaid copending patent application.

A certified absolute standard DC source 32 has its positive side connected to a normally open switch 33. When switch 33 is closed, standard cell 32 is connected to a variable tap arm 34 of resistor 16. The negative side of standard 0611 32 is in series with a null-galvanometer 35 which is connected to divider terminal 21.

Potentiometer 9 must be calibrated in order to use same. For such calibration, ganged switches 24 and 29 are connected to their respective b positions, and switch 33 is temporarily closed. By using preselected values for the standardizing resistors 16, 17, 18 and knowing the exact value of the certified standard cell 32, a 1 ppm. precision voltage of 1.000000 volt is established across voltage divider terminals 20, 21. For example, if cell 32 is certified at 1.01830 volts, the series resistance sum starting from slide contact 34 and going to the right (FIG. 1) through resistor 16 plus resistors 17 and 18 is set at 1830 ohms, whereby the series resistance sum of said 1830 ohms plus 100,000 ohms (divider resistance 19) provides the potential drop across which certified cell 32 is bucked for a null. If the foregoing bucking voltages match, source 10 will be feeding a current of 10 microamps through the foregoing mentioned series resistances and this is indicated by meter 35 nulling. If meter 35 does not null, amplifier resistance 13 is adjusted to establish a null at meter 35. Furthermore, by selecting the resistance value of standardizing resistors 16, 17, 18 to be relatively small in comparison to the input impedance of divider 19, the inherent error of the potentiometer system is elfectively attenuated; said copending application illustrates the details of this point.

The illustrated circuit permits self-calibration of voltage divider 19 to compensate for scaling errors. Since the voltage of standard cell 32 is larger than the voltage established across divider 19, a simple galvanometer comparison is not suitable. A resistor 30 of one-tenth the value of the input impedance of voltage divider 19 is connected in series with voltage divider 19. For divider scaling error calibration, ganged switches 24, 29 are set in positions b, whereby the same current flows through resistor 30 and divider 19. Considering the voltage drop from terminal 20 through divider 19 and through resistor 30 to junction 31, one-tenth the voltage is impressed across resistor 30 and nine-tenths drop will appear across terminals 20, 21. Without disturbing the calibrating setting of supply 10 and the setting of standardizing resistors 16, 17, 18, the standard cell 32 in series with galvanometer 35 is connected at one end to arm terminal 22 and at the other end to junction 31. This is depicted by the arrowheads and dashed lines in FIG. 1. If standard cell 32 is certified at 1.01830 volts, the dial setting of voltage divider 19 should be 0.918300 to null galvanometer 35. Any deviation from 918300 in the divider reading will indicate the scaling error of the system. Refer to the aforesaid copending application for further formulas and explanations of the standardizing and self-calibration techniques employed for potentiometer system 9.

Constant current supply 10 includes the feedback network of amplifier 12, reference voltage 11 and current regulating resistors 13, 14. At a set and operating equilibrium, generator 12 adjusts itself to almost a zero input voltage. Thus, value of the supplied current I times the value of resistors 13 plus 14 must be equal to the reference voltage, or

The values of E and R +R are selected so that I is regulated to be 10 microamps.

During potentiometer use, i.e. measuring and calibrating procedures, ganged switches 24 and 29 are set in positions a and switch 33 is open. Essentially, this reverses divider 19, whereby the fixed 10 microamps supply current flows into divider arm 22 while a load resistor 26 is connected across divider input terminals 20, 21. The potentiometer output E is taken across the load resistor potential terminals 28a, 28b.

In order to understand the operation of potentiometer 9, divider 19 and load 26 are shown in FIG. 3 as an equivalent T circuit. The three divider terminals are the divider input side 20, arm 22 and the low side 21. The values of the equivalent resistances A, B, C are:

where K is the divider ratio, such that when arm 22 contacts terminal 21 then K=0; and when arm 22 contacts terminal 20 then K=1;

Z is the divider input impedance with output open;

and

Z is the divider output impedance with input shorted.

To check the values of A, B and C, we use Thevenins theorem. The total input impedance between terminals 20, 21 with terminal 22 open is:

The output impedance with the input short circuited is between terminals 21, 22 with terminals 20 and 21 The last term is added to the value of B to provide In the instant invention, a fixed value current I is fed into divider arm terminal 22. A load impedance 26 (Z is connected across divider input terminals 20, 21. The output voltage E is taken across load impedance terminals 28a, 28b. The following development employing Thevenins theorem provides the value of E The value of Z the divider input impedance, from terminal 20 to terminal 21 and the values of parameters Zload and I are preselected fixed constants. First open circuit terminals 20, 21. The voltage across the equivalent resistance C is I KZ The circuit to the left of equivalent resistance A now can be considered as a generator whose voltage is K Z I. The load Z (Z is now connected across terminals 20, 21. Looking into divider terminals 20, 21, the Z sees an equivalent generator voltage KZ, I, having a source impedance of The voltage B is then:

Z load 'ZZQm 2) since Z Zload and I are preselected constants, E is directly proportional to the divider transfer ratio K.

E0111; C K As an example, let:

Z 100,000 ohms Z =1,010.1O1 ohms, and I: 10 microamps then, from Equation 2:

E K 1O5 10-5 1,010.101 100,000+1,010.101

The foregoing value of B represents a full scale range of 0.01 volt. Since the accuracy of divider 19 is 1 p.p.m., the maximum potentiometer error will be 0.01 microvolt.

The output impedance of potentiometer 9 is the parallel combination of the Zjoad and the Z whereby the example establishes Z =1000 ohms. This is a relatively very low impedance, which impedance remains constant. By reason of such parameters, the resulting Johnson noise will hardly affect microvolt measurements.

In microvolt potentiometers, the thermal EMFs produced by the system present a substantial problem. With the described circuit, the thermal EMFs produced are substantially reduced. Referring to FIG. 3, thermal EMFs are regarded as originating in divider 19 in association with each equivalent impedance A, B, C, whereby such EMFs are referred to as E E E respectively. These thermal EMFs are considered as being voltage generators in series with the respective equivalent impedances A, B, C. Voltage B is in series with the infinite source feeding terminals 21, 22 and thus may be disregarded. The total thermal voltage E is thus a resultant of the individual EMFs, E and E Such resultant voltage E may be ascertained from Thevenins theorem; in this case the Thevenin voltage is (E -PE and its im- =0.01K volts pedance is A+C=Z This is essentially Equation 2 with the value of the equivalent generator now E +E Using the parameter values previously given, the resultant voltage E across the potentiometer output produced by the internal thermal generators is expressed as follows:

Z loud 1 m wo 4) Equation 4 shows that a potentiometer system in accordance with the invention will attenuate the divider thermal EMFs to .01 of its value. For example, a divider thermal EMF of 0.1 microvolt is thus reduced to 0.001 microvolt at the output.

A second embodiment of the instant invention is shown as potentiometer 39 in FIG. 2. Constant current supply 10 includes high gain amplifier 12, current regulating resistors 13, 41, 42, switch 43 and a feedback loop involving reference voltage 11. Switch 43 is normally in one or the other of its a or b positions in order to complete the circuit to divider 19. Resistors 41, 42 are designed to be switched alternately in series with resistor 13 by switch means 43. Reference voltage 11 has its negative side connected to the input of amplifier 12. The positive side of reference voltage 11 is connected to junction 45. The amplifier common is in series with resistor 13. The output of amplifier 12 is connected to divider arm terminal 22 when the first blade of ganged switch 40 is in its a position.

Current supply 10 is regulated to supply a fixed value current I of microamps to divider 19. The standardizing circuit includes certified DC supply 32 and null meter 35 connected in series from a junction 56 to a second blade of ganged switch 40. A standardizing resistor 44 is in series between divider terminal 21 and junction 56, whereby current I flows through resistor 44. Divider terminals 20, 21 are connected to respective current yoke connections 27a, 27b of load impedance 26. Load impedance 26 is a tapped output resistor having four sections 26a, 26b, 26c and 26d. The values of these sections are:

R =l0,000 ohms R =100 ohms R2 =1,000 OhIIlS R2 11.11 ohms thereby providing a total value of R of 11,111.11 ohms. The yoke voltage output terminals include the low side terminal 46 and individual ones of the tap terminals 47a, 48a, 49a and 50a. Individual load compensation resistors 51, 52, 53, 54 are connected between correlated a voltage output terminals and correspondingly numbered b voltage output terminals, such as 47b, 48b, etc.

The concept of the potentiometer system 39 depicted in FIG. 2 is similar to the previously described system 9. Voltage divider 19 is reversed so that during operation the input driving current I feeds through divider arm 22, whereas the divider output voltage is taken from divider end terminals 20, 21. A load impedance 26 of fixed and selected value of 11,111.11 ohms is connected across divider end terminals 20, 21. The result provides a potentiometer system characterized by reduced thermal EMFs and at low reading linear scale.

In the FIG. 2 embodiment, divider 19 is standardized by connecting switch 40 to its b positions. For this arrangement, source current I is connected to divider terminal 20 and the series combination of cell 32 and null meter 35 are shunted across junction 56 and terminal 20. Assuming cell 32 to be certified at 1.01830 volts, and knowing that an input divider impedance of 100K ohms is shunted by R =11,1l1.l1 ohms, resistor 44 is adjusted to provide 183 ohms since such resistance is in series with the parallel combination of 11,111.11 ohms and 100K ohms. The total of resistance from terminal 20 to junction 56 is 10,183 ohms, thereby establishing 1:1.01830 volts/10,183 ohms=l00 microamps. During the foregoing, it is assured that switch 43 is in its b position. A driving current I of 100 microamps esstablishes EH1: (EA+EC) switch 40 may be set in its a positions thereby connecting m the output of amplifier 12 directly to divider arm 22 and also for connecting the standardizing circuit, the series combination of cell 32 and meter 35 in shunt across resistor 44. For this arrangement, resistor 44 is adjusted to provide 10,183 ohms for a current flow of 100 microamps through said resistor. Cell 32 will buck such voltage when switched into its a position, thereby providing continuous monitoring of potentiometer system 39 during its operation. Since certified cells are within the range of 1.01700 and 1.02000 volts, resistor 44 is selected to provide two ranges of resistance values. The first range is between 170 ohms to 200 ohms to cover the initial standardizing procedure. The second range is between 10,170 ohms to 10,200 ohms to provide continuous monitoring of potentiometer 39 during use of same.

FIG. 2a through 20 illustrated an embodiment of FIG. 2, wherein resistor 44 is replaced by two variable resistor 44a and 44b in series, respectively, with divider terminals 20, 21. Resistor 44a is a variable in the range from 170 ohms to 200 ohms and is essentially the equivalent of resistor 44 of the FIG. 2 embodiment. A normally open shorting switch S appears across resistor 44b.

FIG. 2a depicts calibrating and standardizing potentiometer 39 with 1 volt, 1 p.p.m., across divider terminals 20, 21 by bucking the drop from junction 56a to divider terminal 21 with the series combination of cell 32 and null meter 35. Switch S is closed to short out resistor 44b. Resistor 44a is set at 183 ohms for a certified cell 32 as previously specified.

FIG. 2b illustrates calibrating divider 19 for scaling errors after 1 volt is established across its terminals 20,

21. In this case, the series combination of cell 32'and meter 35 is connected across divider arm 22 and junction 56. Resistor 44b is set to provide a voltage drop across same which is a known percentage of the drop across divider terminals 20-21 and thus serves as resistor 30 in the FIG. 1 embodiment.

Finally, FIG. 20 illustrates the arrangement for continuously monitoring divider 19 during its operation by sensing the voltage across resistor 44b which is now set to provide 10,183 ohms for the reasons described hereinbefore.

The full scale output voltage B from terminal 46 to terminal 47a can be determined from Equation 2:

Z OB

where 2, 100,000 ohms, Z =11,111.11 ohms, I=100 microamps, and K=1 for full scale; therefore, E t l VOlt.

Similarly, the full scale E can be calculated for the other resistor sections. For example, if the output is taken between 48a and 46, the full scale output voltage is .1 volt; between 49a and 46, E full scale is .01 volt; from 50a to 46, full scale output voltage is .001 volt. Since voltage divider 19 is accurate to 1 ppm, the lowest range provides reading values to .001 microvolt.

When the illustrated microvolt otentiometers are used with an external load, such as a digital voltmeter 55, the problem of source load impedance arises. In the instant invention, output impedance Z is a constant and equal to the parallel combination of the voltage divider input impedance Z and load impedance Z as shown in Equation 3. The error due to loading efiects can be eliminated by using appropriate load compensation resistors. FIG. 2 illustrates load compensation resistors 51, 52, 53, 54 at the output of potentiometer 39. The values of these load compensation resistors are individually selected to equal the output terminal impedance at the taps the output is being takerr For E between terminals 47a and 46, the total output impedance is:

out

out out+ L is equal to or less than 0.001, where Z is the output impedance of the potentiometer system looking into the resistor 26 tap terminals across which the external load, such as voltmeter 55, is connected; and Z the value of the impedance of the external load, such as voltmeter 55. By the above definition, Z may be considered as the source impedance of potentiometer 39 to which a load Z is connected.

Consider the following illustration as outlining a procedure for achieving automatic load compensating upon applying an external, digital voltmeter 55, to the tapped output of resistor 26. Divider arm 22 is moved to terminal end 20 so that K=1. Initially, switch 43 is set at its b position. The potentiometer system is standardized and current supply 10 is regulated to deliver the fixed value 100 microamps as described hereinbefore. Load 55 is connected to the tapped output with one lead 57 thereof connected to terminal 46 and the other leads 58 connected to the desired one of the a voltage output terminals, such as 47a or 48a, etc. A voltmeter reading is taken and its value is noted. Load lead 58 is then connected to the correlated b output terminal (shown in dashed outline), for example, if the upper lead 58 was at 47a, then the upper lead 58 is then connected to 47b. This inserts resistor 51 in the potentiometer circuit. Switch 43 is now set in its a position and current booster resistor 41 is adjusted until the b voltmeter output reading equals the prior a output reading. This automatically compensates the output reading as to load errors. Allowing switch 43 to remain in its a positon and without disturbing the adjustment to resistor 41, upper load lead 58 is returned to connect with the original a output terminal. All further measurements and external load sensing against the calibrated potentiometer for the scale represented by output terminals 46-47a are now taken from such terminals 46-47a because the potentiometer circuit is now automatically load compensated. As an example, assume the voltmeter reading at terminal a is 0.9998 volt; and at terminal b, the reading before adjustment is 0.9997 volt. For the b reading, the source impedance has been doubled because of the introduction of load compensator 51. The resultant error upon doubling the source impedance is 0.0001 volt. This indicates an initial loading error of 0.0001 volt due to potentiometer loading. After adjust ment and upon returning the voltmeter to terminal a, if its reading is 011 1.00000 volt this indicates a scaling error for the voltmeter.

It will be understood that the embodiment of FIG. 1 also may include the foregoing tapped resistor 26, load compensating resistors 51 through 54, and current reg- 9 ulating resistor 41 at supply 10 for the purposes as described with respect to FIG. 2.

The instant invention can be seen in the illustrative block diagram form in FIG. 4, The potentiometer consists of a current source 10 designed to supply a constant current I to arm 22 of voltage divider 19. The end terminals 20, 21 of voltage divider 19 are connected to a load impedance 26. Load impedance 26 must be constant, but may be either a variable or tapped resistor. Although the description has been made using a three terminal voltage divider, the invention is not limited to its use. Any linear transfer ratio device with constant input impedance can be used.

The foregoing low thermal potentiometer systems have been designed and compensated to cancel out and attenuate spurious thermal EMFs. In order to secure a continuity of precision low voltage operation at the external load end of the circuit during use of the disclosed potentiometer systems, the problem of external wiring efiects are also contemplated herein. Some wiring materials heat up at a faster rate or to higher temperatures than others, thereby generating thermal gradient EMFs. External connecting wires and switch assemblies are affected by differences in room temperatures and other ambient variations. Such phenomenon causes one wire to become hotter than another causing the generation of spurious thermal EMFs. This is of principal concern with regard to measurements or calibrations of unknown voltages when dealing with high precision voltages in the low value range. To some extent, the thermal EMF problem may be mitigated by the use of similar conducting material throughout the circuit, for example, one uses identical copper wiring throughout the entire system. However, for the very sensitive high precision system operation as contemplated herein, the use of heretofore known prior techniques is not satisfactory.

The instant invention contemplates multiwire connections, wherein these wires are arrayed in geometric patterns in accordance with the principle of achieving thermal electrical center of gravity connections. For example, in FIG. 2, meter 55 is connected by tWo leads 57, 58 to the potentiometer output. The potentiometer circuit and a substantial portion of leads 57, 58 may be encased in a thermally controlled environment. However, small portions of leads 57, 58 and meter 55 will be exposed to ambient environment and thus exposed to ambient thermal fields. As another example, in FIG. 4, a voltage source 60 under test is in series with a null meter 61. The series combination is connected across the output of the potentiometer system for the purpose of ascertaining or calibrating the voltage of source 60. Setting of divider dial 23 is adjusted until meter 61 nulls. The measured voltage value of source 60 is determined by the setting of dial 23. Here again, the external load connection wires to the greatest extent possible are placed in a thermally controlled environment with the potentiometer system. For example, everything to the left of dash line 6262 is in such controlled environment, whereas short sections of leads 57', 58' and null meter 61 are exposed to ambient fluctuations and thermal fields. Assume a thermal field flowing in the direction depicted in FIG. 4 by arrows F1 and F2 envelopes lead portions 57, 58' and that the thermal field F1 is hotter than at F2, this causes a thermal gradient to exist between single leads 57' and 58. As a result, thermal EMFs are generated in these wires, which EMFs are spurious output signals sensed by meter 61.

FIG. 5 illustrates the principle of the thermal center of gravity arrangement of multiple connection wires. Battery B is connected to a meter or external load M. The interconnected terminals a-a' are actually connected by a pair of lengthwise wires A, A. Similarly, interconnected terminals b-b' are connected by a pair of lengthwise wires B, B. The cross-sectional space arrangement or order of the Wires is depicted by FIG. 6 showing the wires in an array for a plane (depicted by dashed line P) perpendicular to the lengthwise axis of wires A and B. The wires are grouped so that each A wire is suitably spaced symmetrically with respect to the pair of B wires;

taken as a pair with the net result that there is no resultant t thermal gradient between the interconnections from terminals a, a to terminals b, b. Essentially, the invention contemplates multiple wire connections and terminations arranged geometrically in a pattern so as to be symmetrically disposed in the thermal fields enveloping same, whereby there is a cancellation of temperature gradient thermal EMFs over external connections between two electrical devices.

FIG. 7 illustrates the connection of source or unknown 60 connected in series with meter 61 to the output of the potentiometer system with everything to the left of dashed line 62' in a thermally controlled environment and the circuit to the right of dashed line 62 in the ambient atmosphere. In this instance, a polarity reversing switch 63 (shown in dashed outline in FIG. 7) is inserted in the external load circuit to connect the four wire lines leading to 61 in accordance with the principles taught in FIG. 5.

FIGS. 8 through 11 illustrate an embodiment of the thermal center of gravity and polarity reversing switch 63. Switch 63 is made up of a first group of four lengthwise conducting wires C-D, C-D and a second group of four lengthwise conducting wires ]-K, ]K. The two wire groups are supported in tandem relationship by respective pairs of axially spaced disc-shaped members 64, 65 and 66, 67. Members 64, 65 and 66, 67 are made of electrical insulator material to avoid shorting the switch wires. The switch wires are encased in a substantially enclosed isothermal shield container which for convenience consists of a cylindrical metallic sleeve 68. The opposite ends of sleeve 68 are enclosed by respective members 64, 67. The outer end portions of the wires of the respective groups are rigidly supported and held in fixed position by correlated end members 64, 67. These wires extend through correlated members 64, 67 and are provided with external terminal posts 69 to which electrical' connections are made.

The inner end portions of the two groups of wires pass through individual arcuate slots 70, 71 of respective closely spaced apart and confronting members 65, 66. Members 65, 66 are individually mounted for threaded screw turning on an axial shaft 72. Shaft 72 is rigidly supported by end members 64, 67. Shaft 72 is threaded at 73 to permit screw turning of either member 65, 66 upon selective actuation of switch 63.

When switch 63 is in a non-conducting status, each of the four wires in the 0-D group is held by supporting members 64, 65 in straight lengthwise and parallel relationship with respect to each other. In cross-section, i.e., a plane perpendicular to the axis of shaft 72, the individual four C-D wires are each geometrically arrayed to lie at an individual corner of a square, but at respective radial lines of 45, 225 and 315". This array is depicted in FIG. 9. Similarly, the four wires of the IK group are also held in lengthwise straight and parallel relationship with respect to each other. However, in the cross-sectional plane, the individual J-K wires are held by members 66, 67 in a symmetrical array lying along respective radials of 0, 90, 180 and 270 as depicted in FIG. 10. Since it is assumed that switch '63 is isolated from thermal gradient fields by its isothermal container, the cross-sectional arrays of its two wire groups are both geometrically symmetrical. However, it is noted that the I-K wires are angularly offset 45 with respect to individual ones of a pair of -D wires as seen in FIG. 11. At the center of switch 63, that is to say, in the region of the plane taken along line 74-74 of FIG. 8, the inner ends of the individual wires of one group axially overlap the inner ends of the Wires of the other group to permit switching connection therebetween upon actuation of switch 63. However, there is no contact between any one wire of one group with a wire of the other group for the non-connecting status of switch 63. This is depicted in FIG. 11. It is seen that the foregoing switch arrays locate the eight wires along the same circumference about the axis of shaft 72.

By reason of the foregoing described cross-sectional array of switch wires, the inner end of each wire of one group is in juxtaposed and mutually spaced apart relationship between inner ends of a pair of wires of the other group without contact being made therebetween. This defines the non-connecting status of switch 63.

Switch actuating means 75 is mounted at a sleeve opening 76. Actuating means 75 has a flange 77 for enclosing opening 76. Means 75 includes a member 78 extending between members 65, 66 and mounted for rotation about its vertical axis upon turning an exterior knob '79. A horizontal rod is integrally carried at the inner end of member 78, which rod has oppositely extending ends 80, 81. Rod ends 80, 81 at all times are in driving engagement with a wall of individual recesses 82, 83 of respective members 65, 66, as shown in FIGS. 9 and 10. When knob 79 is turned in one direction about its vertical axis, rod end 80 drives against its adjacent recessed wall to turn disc 65 counterclockwise as viewed in FIG. 9. This action simultaneously twists the free end portions of the four C-D wires counterclockwise about the axis of shaft 72 until each C wire electrically contacts a stationary adjacent K wire. Simultaneously, each D wire electrically contacts a stationary I wire. A 45 turning movement of member 65, as depicted by the four full line arrows in FIG. 11, achieves the foregoing switching connection. The foregoing movement does not disturb the stationary status of member 66 because the originating turning of knob 79 drives rod end 81 to the right, as seen in FIG. 10, through the open clearance of its slot 83. Turning of knob 79 in the opposite direction drives rod end 81 to the left, as seen in FIG. 10, to rotate member 66 counterclockwise about shaft 72 until the free end of each I wire connects with an adjacent stationary C wire, while the free end of each K wire connects with a stationary D wire. This switching action is depicted by the four dash line arrows in FIG. 11. The foregoing knob turning does not disturb member 65 because rod end 80 is driven through the open clearance of its recesses 82, as viewed in FIG. 9.

To assure good switch contact between juxtaposed adjacent group wires, the inner ends of the wires may be suitably bent or shaped. When switch 63 is in one or the other of its connecting phases, the free inner ends of one group of wires are essentially twisted about the axis of shaft 72 to effect the switch connections, whereby the corresponding member 65-66 turns along shaft thread 73. As seen, switch wires are essentially cantilever supported from their respective rigid supports 64, 67. The spring force developed by the twisted wires spring loads the wires to assist returning switch 63 to its non-connecting status upon release of knob 79. It will be understood that the switch wires are selected to have sufficient elasticity and resilience so as to return to their respective axially straight and parallel relationship each time knob 79 return to a non-connecting status. Furthermore, the dimensions of slots 70', 71 are selected to provide suitable clearance for respective switch wires extending through same to permit the foregoing switching action to take place and to assure freedom of binding of the coacting moving parts. From FIGS. 9 and 10, it is seen that the switch wires are nested at the ends of the respective slots 70, 71 so as to undergo twisting upon turning of the respective members 65, 66.

Although knob 79 is manually operated herein, a system may be devised for actuating knob 79 by a servo signal. The foregoing embodiment contemplates a 45 turn of each member 65, 66 in order to drive switch 63 into a connection status. If the respective recesses 82, 83 are made narrow, that is to say, without the disclosed open clearances for rod ends 80, 81, so that both members 65, 66 turn simultaneously upon knob turning but in opposite directions about shaft 72 as viewed in FIGS. 9 and 10, each member 65, 66 need turn only 225 to effect a switch connection status. This offers advantages of requiring less rotational travel for each member 65, 66 and balances the spring load on the actuating means for a switch connection.

In FIG. 7, consider switch 63 connected into the four wire connections between potentiometer source external terminals a-a' and load terminals b-b'. Assume that the individual wires from terminal a are connected to respective C, C switch wires, those from terminal a are connected to the D, D switch wires; and on the other side, the wires from terminal b are connected to the K, K switch wires and those from terminal b are connected to the J, I switch wires. This is shown schematically in FIG. 11. For the switching connection as depicted by movement of member 65 counterclockwise as viewed in FIG. 9, the connection is essentially that shown in FIG. 11 by the four solid line arrows, whereby terminal a connects to terminal b and terminal a connects to terminal b through switch 63. For the reverse switching action depicted by the four dash line arrows in FIG. 11, terminal a is now connected to terminal b', and terminal a is now connected to terminal b. Hence, switch 63 reverses the polarity of the DC connection of member 61 to sense an output measurement. If both of the foregoing meter 61 readings are equal, the thermal gradient EMFs are essentially all cancelled out. If the second reading differs from the first reading, one-half of the difference of the two readings is the amount of residual thermal gradient EMFs in the exterior output circuit. Upn release of knob 79, switch 63 returns to its normally open switch status and thus output power is drawn from the potentiometer system only during an actual switching connection.

What is claimed is:

1. Potentiometer means useable for precision microvolt range measurements and providing standardizedprecision low voltages effectively free of spurious random noise and thermal EMFs comprising, means providing a constant current source, linear transfer ratio means having at least three terminals for defining an input and output thereof and characterized by a predetermined and fixed value of input impedance at first and second of said terminals and a selectable output impedance at a third of said terminals in accordance with said linear transfer ratio, means for varying said transfer ratio, means for connecting a load of selected and fixed impedance to the input of said transfer means, means including said source for applying a current of selected fixed value to said transfer means for establishing a standardized predetermined potential difference across said transfer means input, and means for applying said constant current source to the output of said transfer means for establishing standardized predetermined values of voltage potential across said load directly proportional to the transfer ratio of said transfer means.

2. Potentiometer means as defined in claim 1, said load having voltage output terminals for providing an output measurement, said potentiometer serving as a source having a source impedance looking into said voltage output terminals upon connecting an external load across same, means for connecting an external load to said voltage output terminals and for providing a measurement of the output voltage sensed by said external load, means for connecting a load compensation impedance in series with one of said voltage output terminals and said external load for providing a measurement of output voltage sensed by said external load with the potentiometer source impedance increased by said lead compensation impedance, the value of said load compensation impedance being equal to the source impedance of said potentiometer system looking into said pair of voltage output terminals, and means for regulating said constant current source to adjust the second measured output voltage to equal the first measured output voltage if a difference exists therebetween, wherein said potentiometer means is automatically compensated for loading effect errors introduced upon connecting said external load across said voltage output terminals.

3. Potentiometer means as defined in claim 1, said load having voltage output terminals for providing an output measurement, wherein said potentiometer serving as a source having a source impedance Z looking into said voltage output terminals for an external load connected across same, means for connecting an external load to said voltage output terminals and for providing a meas urement of the output voltage sensed by said external load, means for connecting a load compensation impedance in series with one of said voltage output terminals and said external load for providing a measurement of output voltage sensed by said external load with the potentiometer source impedance increased by the value of said load compensation impedance, and means for regulating said constant current source to adjust the second measured output voltage to equal the first measured output voltage if a difference exists therebetween, wherein said potentiometer means is automatically compensated for loading effect errors introduced upon connecting said external load across said voltage output terminals.

4. Means as defined in claim 3, wherein the ratio of Z (Z -l-Z is in the order of equal to or less than 0.001

for the purpose of retaining a very high degree of preci sion potentiometer operation.

5. A potentiometer system useable for precision microvolt range measurements and calibration comprising, a constant current supply, voltage transfer means having a constant impedance input and variable output impedance related to said input by a linear transfer ratio, first means including said supply for establishing a standardized voltage at the input of said transfer means, second means for connecting said input of said transfer means in a closed circuit, and third means for connecting said current supply to the variable output of said transfer means, wherein output voltages established at said input are a direct function of said linear transfer ratio.

6. A system as defined in claim 5, wherein said first means including, a circuit means for providing a reference voltage for establishing said standard voltage across said transfer means input, first impedance means of selected value in series with the input of said transfer means for establishing a given value of constant current for standardizing said transfer means, and second impedance means in series with said transfer means for monitoring said supply constant current fed to said transfer means.

7. In a potentiometer system as defined in claim 5 further comprising, a fixed impedance load in said closed circuit having voltage output terminals for sensing output voltages, an external load connected to said voltage output terminals, means including respective sets of plurality of wires for nullifying the effects of thermal field gradients in low voltage electrical connections between said output voltage terminals and said external load, said external load having a plurality of terminals'for electrical connection with individual ones of said voltage output terminals by said respective sets of a plurality of wires, each set including a wire connection for the interconnected terminals between said external load and voltage output terminals, each wire of one set being in symmetrical spaced arrangement regarding a wire of the other set with respect to thermal fields passing over said wires to compensate for thermal gradient effects produced in the individual connections between said external load and potentiometer system.

8. A potentiometer system useable for precision microvolt range measurements and calibrations comprising, a constant current supply, voltage divider transfer means having atleast three terminals for providing a constant impedance input and a variable output impedance related to said input by a linear transfer ratio, means having a selected value of impedance in series with said supply for establishing a standardized voltage across the input of said voltage divider means, a load of fixed value impedance, means for connecting said load to the input of said divider, and means for connecting said current supply to the divider variable output, wherein voltages established at said load are a direct function of said linear transfer ratio.

9. A potentiometer system useable for precision microvolt range measurements and calibrations comprising, a predetermined current supply presenting an essential infinite impedance, voltage transfer means of at least three terminals and having at least a pair of constant impedance terminals and at least a pair of variable impedance terminals such that there is a predetermined transfer ratio between the said pairs of terminals, means for connecting said constant impedance terminals of said transfer means in a closed circuit, and means connecting said current supply to the variable impedance terminals of said transfer means, wherein output voltages established at said constant impedance terminals are a direct function of said transfer ratio.

References (Iited UNITED STATES PATENTS 1,625,125 4/1927 Latour 174-34 2,119,364 5/1938 Smith 324- 2,880,393 3/1959 Cornish 32499 XR 2,958,724 11/1960 Milloit 174-34 XR 3,267,374 8/1966 McAdam et a1. 324-415 XR RUDOLPH V. ROLINEC, Primary Examiner. ERNEST F. KARLSEN, Assistant Examiner.

US. Cl. X.R. 323-74 V 

